Sinus tachycardia is basically a heart rate above 100 that is not an arrhythmia, in other words, a normal rhythm, but rapid.
As Tom said, there are lots of causes for sinus tachycardia. Some are normal, some are caused by an underlying disease that actually rarely has something to do with the heart.
Normal causes of sinus tachycardia:
- Exercise / physical activity (incl. activities like climbing stairs, sex, etc)
- Stress
- Anxiety
- Emotions
- After heavy meals, especially high-carb meals.
- Standing up too quickly
- Deconditioning
Non-cardiovascular disease causes for sinus tachycardia
- Dehydration (usually caused by low blood pressure)
- Hypoglucemia
- Hyperthyroidea
- Fever
- Pheocromocytoma, carcinoid, and other hormonal disturbances affecting stress hormones (extremely uncommon)
- Infections
- Electrolyte disturbances
- Overactive nervous system.
Cardiovascular causes:
- Shock / severe allergic reactions
- Low blood pressure (usually not dangerous)
- Heart failure
- Heavy bleeding
- Pulmonary embolism
(most of those would cause a lot of symptoms and tachycardia would be the least concern)
If NO cause is found and the heart rate never drops below 100, except during sleep, diagnosis may be "inappropriate sinus tachycardia".
Depending on how rapid the heart rate is, your doctor can decide to treat the condition, or the underlying condition. But sinus tachycardia is usually a normal response.
Sinus Tachycardia is a normal physiological response to situations such as exercise, or an increased sympathetic tone with increased catecholamine release—stress, fright, flight, anger. Other causes include:anxiety, fever, dehydration, etc. IOW, it's normal. Now.... there are other types of tachycardia which are NOT normal; ventricular tachycardia, supraventricular tachycardia, inappropriate sinus tachycardia, postural orthostatic tachycardia. Do you have the nomenclature correct?